The present invention is directed toward control systems. More specifically, this invention is directed to a control system for turbo machinery.
Turbo machinery such as compressor systems like that seen in FIG. 1 of U.S. Pat. No. 5,798,941 that is incorporated herewith have been around for many years. Typically, a complex control system is used in order to control a recycling valve that regulates fluid flow to a compressor. Industrial control systems including turbo machinery control systems use methods of reading inputs and calculating control response. These responses are often analog signals that need to be sent to a control device. The control signal sent to the control device (in the case of turbo machinery control systems is typically the valve) is an analog current signal typically ranging from 4-20 mA. This analog signal is critical to the proper operation of the compressor surge controller.
The present invention relates in general to an electrical network where analog control signals are used to modulate a valve, pump or motor and more specifically the use of this analog network to control such devices in a turbo machinery system. Many turbo machinery control systems require that the input and output signals be highly reliable. The previous methods have included: (1) using redundant inputs but leaving the analog outputs as simplex; (2) using redundant analog outputs in parallel and wired through an electromechanical relay. The electromechanical relay is switched to allow either controller A or controller B to be driving the device; (3) Other more complex methods have been used to create feedback methods and diagnostics to monitor the signal to the field and the backup signal as best shown in U.S. Pat. No. 4,652,417.
Despite these solutions problems remain. The use of simplex analog outputs is a low cost solution but does not provide any level of fault tolerance in the case of electronic component failures in the analog output circuitry. The use of electromechanical relays to switch between the analog signals is a more reliable solution, but has the problem of not being able to correctly determine when an output channel has failed. These methods use feedback signals from the analog output that are measured before the field load device as best seen in U.S. Pat. No. 5,508,910. If the failure of the analog output circuit is such that the analog signal is grounded before the field load, but after the feedback measurement, then the circuit will fail to identify the failure.
Thus, a principal object of the present invention is to provide a redundant analog output signal to a field device.
Yet another object of the present invention is to utilize standard PLC components to provide a reliable output system.
Yet another object of the present invention is to detect failures that occur after loading in a compressor system.
These and other objects, features, or advantages of the present invention will become apparent from the specification and claims.